Formation and acceleration of uniformly filled ellipsoidal electron bunches obtained via space-charge-driven expansion from a cesium-telluride photocathode

We report the experimental generation, acceleration, and characterization of a uniformly filled electron bunch obtained via space-charge-driven expansion (often referred to as “blow-out regime”) in an L-band (1.3-GHz) radiofrequency photoinjector. The beam is photoemitted from a cesium-telluride semiconductor photocathode using a short ($<200\mathrm{fs}$) ultraviolet laser pulse. The produced electron bunches are characterized with conventional diagnostics and the signatures of their ellipsoidal character are observed. We especially demonstrate the production of ellipsoidal bunches with charges up to $\sim 0.5\mathrm{nC}$ corresponding to a $\sim 20$-fold increase compared to previous experiments with metallic photocathodes.

Figure 1

Domain of existence of the blow-out regime (lighter colors) in the $({\sigma }_0,E_{\mathrm{acc}})$ parameter space for ${\tau }_l=50\mathrm{fs}$ (a) and ${\tau }_l=200\mathrm{fs}$ (b). The horizontal blue dashed lines correspond to $E_0=35\mathrm{MV}/\mathrm{m}$. (This figure was adapted from Ref. 13.)

Figure 2

Top view of the A0PI setup displaying elements pertinent to the present simulations and experiments. The “L” refers to solenoidal lenses, “X” to diagnostic stations (beam viewers and/or multislit masks location), and “Q” to quadrupole magnets. The distances are in mm and referenced to the photocathode surface.

Figure 3

Characterization of the UV laser pulse. Measured spectrogram (a) using a polarization-gate implementation of the frequency-resolved optical gating (FROG) technique and reconstructed temporal distribution (b) of the UV pulse (the inferred rms UV pulse duration is 122 fs). Typical transverse spatial distribution of the UV laser pulse on the photocathode (c) with associated projection (white traces) and parabolic fits (green traces).

Figure 4

Monte Carlo simulation (squares) taken from Ref. 39 and parametrization (dashed line) of the electron transit time from an initial Dirac-like laser excitation (a). Impact of the slow response time on the photoemission profile (blue traces) for three different Gaussian-like laser profiles (green dashed traces) of indicated rms duration (b). The traces in plot (b) are vertically offset for clarity. Larger time ordinates correspond to the bunch tail.

Figure 5

Simulated spatiotemporal distribution $(z,x)$ at $s=0.47\mathrm{m}$ (a) and $s=3.77\mathrm{m}$ (b) (corresponding to X3 screen) w.r.t. to the photocathode and associated longitudinal charge distributions [(c) and (d), respectively]. The charge is 250 pC and the rms laser transverse size is ${\sigma }_c=1\mathrm{mm}$. In these plots the tail of the bunch is at $z>0$.

Figure 6

Simulated spatiotemporal distribution $(z,x)$ (left column) and associated longitudinal charge distribution (right column) simulated at X3 ($s=3.77\mathrm{m}$) for three cases of charges 100 [(a), (b)], 250 [(c), (d)], and 1000 pC [(e), (f)]. In these plots the tail of the bunch is at $z>0$. Plot (g) illustrates the rms bunch length evolution along the beam line up to $z=3.77\mathrm{m}$ (location of X3) for the three cases of bunch charge. The shaded area in plot (g) indicates the location of the booster cavity. The rms laser transverse size is kept constant to ${\sigma }_c=1\mathrm{mm}$ for the three cases of charges.

Figure 7

Computed ${\mathcal{D}}_z$ metric [see Eq. (4)] (a) for bunch charges of 100, 250, 500, 1000, and 2000 pC (shown respectively as blue, green, red, cyan, and magenta symbols) as a function of cathode surface density ${\sigma }_0$. The gray line indicates loci of minimum ${\mathcal{D}}_z$ and the insets (b), (c), and (d) show typical spatiotemporal distributions simulated for the three regimes delineated by the gray line; see text for details. The simulation results are evaluated at the location of X3 in Fig. 2. The insets plot horizontal (respectively vertical) axes corresponding to $z$ (respectively $y$) both in units of mm.

Figure 8

Examples of measured spatial $(x,y)$ distribution and associated projections (white solid lines) with parabolic fit (green dash lines). Images (a) and (b) respectively correspond to the beam being photoemitted using the ultrashort Ti:Sp and the picosecond Nd:YLF laser systems. The beam density is measured at the Ce:YAG screen X6 (see Fig. 2) for $Q\simeq 250\pm 50\mathrm{pC}$.

Figure 9

Principle of measurement of the LPS chirp [(a), (b), and (c)] and bunch length [(d), (e), and (f)] measurements. The left [plots (a), (d)], middle [plots (b), (e)], and right [plots (c), (f)] columns show respectively the initial LPS, the location of the bunch (shown as a blue circle) w.r.t. the accelerating voltage, and the final LPS. The images (g) and (h) respectively show an example distribution measured at the XS3 location for minimum-energy-spread and off-crest phase settings; see text for details. In these latter images the horizontal axis corresponds to the energy-dispersed direction [$x[\mathrm{mm}]\simeq 317\delta$ where $\delta$ is the unitless fractional momentum spread].

Figure 10

Measured longitudinal-phase-space chirp $\mathcal{C}\equiv ⟨z_0{\delta }_0⟩/⟨z_0^2⟩$ as a function of charge for three cases of laser transverse rms size ${\sigma }_c$ on the photocathode surface. The horizontal dashed line represents the nominal chirp when the 3-ps-long Nd:YLF laser is used for photoemission. The shaded area represents the domain where the parabolic character of the temporal distribution was observed to be distorted and is consequently associated to a regime where a uniformly filled-ellipsoid bunch is not realized. The chirp associated to the electron bunch produced with the Nd:YLF laser was found to be independent of the charge and only one measurement (performed for $Q=250\mathrm{pC}$) is shown.

Figure 11

Measured spatiotemporal $(x\propto t,y)$ distribution and associated projections (white solid lines) with parabolic fit (green dash lines). The beam density is measured at the Ce:YAG screen XS3 (see Fig. 2) for $Q=100\pm 20\mathrm{pC}$ (a) and $Q=480\pm 30\mathrm{pC}$ (b).

Figure 12

Measured peak-normalized temporal profiles for $Q\simeq 130$, 280, 460, and 700 pC (respectively shown as blue, red, green, and cyan circles), compared with the measured uv laser temporal profile (black solid trace). The dashed lines represent the results of parabolic fits to the measured profiles. The head of the bunch corresponds to $t>0$.

Figure 13

Measured rms electron-bunch duration (symbols) as a function of charge for two cases of laser transverse spot size ${\sigma }_c$ on the photocathode. The dashed red and blue lines are the corresponding simulations carried with astra. The dashed cyan line represent the UV laser pulse duration.

Figure 14

Normalized transverse horizontal emittance measured as a function of the current setting of the solenoidal lenses (shown as L1, L2, and L3 in Fig. 2) for $Q=266\pm 25\mathrm{pC}$.